PTC thermistors have been commonly used in self-regulating heaters, and are increasingly employed in electronic devices as components to protect against overcurrent. Exposure to overcurrent in an electric circuit causes the conductive polymer sheet inside a PTC thermistor to heat up and expand. This thermal expansion of the conductive polymer sheet increases the resistance of the PTC thermistor and thus reduces the current to a safer level. There are increasing demands for PTC thermistors that carry high currents, have low resistance, are compact in size, and yield a low voltage drop.
A conventional PTC thermistor is described below.
One known PTC thermistor is disclosed in the Japanese Laid-open Patent No. S61-10203. This PTC thermistor is created by laminating a plurality of alternate layers of conductive polymer sheets and metal foils, with side electrodes on opposing sides.
FIG. 10 is a sectional view of a conventional PTC thermistor. In FIG. 10, a conductive polymer sheet 1 is made of a high polymer material, such as cross-linked polyethylene, and dispersed conductive particles, such as carbon black. An inner electrode 2 is made typically of a sheet of metal foil, and is sandwiched between the conductive polymer sheets 1. The inner electrode 2 is also disposed on the top and bottom of the conductive polymer sheet 1, while leaving a no electrode area 3 at the starting end, portions of the middle and finishing ends of the conductive polymer sheet 1 as shown. Alternate layers of the inner electrode 2 and conductive polymer sheet 1 form a laminated body 4. A side electrode layer 5 forms a leader section, and is disposed at the side of the laminated body 4 so as to be electrically coupled to one end of the inner electrode 2.
However, the conventional PTC thermistor created by laminating the conductive polymer sheet 1 and inner electrode 2 alternately to create low resistance undergoes repetitive expansion and shrinkage of the conductive polymer sheet 1 when an overcurrent condition is created and alleviated. This may cause failure in connections to the side electrode due to cracking generated as a result of stresses generated by the expansion and contraction of the conductive polymer sheet 1.
The present invention aims to provide a highly reliable PTC thermistor with good withstand voltage which eliminates failure in a connection to a side electrode by cracks, and its manufacturing method.